First you need to know what type of bindings you are going with. If you use the strap in bindings than you are good to use any boot, unless it is a step-in boot. A step-in boot is usually firmer and needs to be bought with your step-in bindings.

Boots are made up of two sections, the liner and the shell. The liner's job is to keep your feet comfortable and warm. Better liner's will feature things like heat-molded liners, their own lacing system to maximize fit, and a super cushy foam for more warmth and comfort.

You don't want to settle for the first pair of boots you try on. You should try on a bunch of boots from different companies. Your foot is unique and and so is every boot. You are going to be in your boots for at-least 3-5 hours so you need to make sure they were made for your feet.

Make sure that you wear the same socks that you go riding with when you are trying on your boots. Put your feet into the liners and tighten(some liner's don't tighten and don't separate from the shell, skip next sentence). Then insert the liner into the shell. Make sure your heel is all the way in the back and lace up. Once your all laced up strap into a a board with bindings. This is when you want to test the boots out. Try leaning back and forth & side to side. Things to pay attention to are: Does your heel stay in place, especially when leaning forward, Does your foot slide around, and are there any pressure points that hurt.

Finding the boot with the right amount of flex for you is also important. Your desired boot flex depends upon your riding style. Freestyle riders usually like softer more flexible boots. Freeride boots are typically stiffer and offer more support. Alpine riding boots are hard like ski boots which offer maximum support.